The main olfactory epithelium (MOE) in the mammalian nasal cavity serves two distinct functions. First, the MOE detects almost an infinite number of airborne odor molecules and initiates the sense of smell. Second, the MOE metabolizes and removes inhaled xenobiotics, which include high levels of odorous irritants, air pollutants and microorganisms. This latter, epithelial defense function is critically important, no only for maintaining the proper function of the MOE, but also for protecting vital organs, such as the lower airway, lungs and brain by limiting xenobiotic access to these organs. However, fundamental information about mechanisms of xenobiotic detection and the subsequent pathways coordinating MOE defense is missing, which limits our understanding of olfactory dysfunction in respiratory diseases associated with xenobiotic insults. We recently have identified a population of transient receptor potential M5-expressing microvillous cells (trpM5-MCs) in the MOE. Our initial study shows that trpM5-MCs are chemosensitive and cholinergic, meaning they are capable of releasing acetylcholine (ACh) to modulate intracellular Ca2+ levels of neighboring supporting cells. We hypothesize that trpM5-MCs detect xenobiotics and subsequently coordinate activities of the MOE defense network. We will test this central hypothesis by pursuing the following four specific aims. Aim 1 will determine the response profiles of trpM5-MCs and the key elements necessary for xenobiotic detection. Aims 2 and 3 will characterize intracellular and intercellular pathways that allow trpM5-MCs to coordinate and regulate MOE defense network activities. Aim 4 will determine morphological and functional deficits in the MOE of mice with defective trpM5-MCs. The rationale for the proposed research is that knowledge of the mechanisms of xenobiotic detection and the pathways leading to regulation of xenobiotic clearance is important for the understanding of epithelial defense and will enable innovative approaches to target the initial development of xenobiotic-associated respiratory diseases. We expect to obtain fundamental knowledge about xenobiotic detection mechanisms and subsequent pathways that coordinate and regulate MOE defense. We also expect that defects in xenobiotic detection in trpM5-MCs will impair the MOE defense network, resulting in structural and functional deficits in the epithelium. This proposed research is innovative, because it represents a new approach to understand the relationship between xenobiotic exposure and respiratory diseases. The significance of these studies is that they lay the groundwork for future research on xenobiotic-induced pathological changes that may ultimately lead to new treatments for xenobiotic exposure-related olfactory dysfunction and respiratory diseases.